The drilosphere concept: Fine-scale incorporation of surface residue-derived N and C around natural Lumbricus terrestris burrows

Anecic (deep-burrowing) earthworms are important for soil biogeochemical functioning, but the fine-scale spatial range at which they incorporate C and N around their burrows (the drilosphere sensu stricto) needs to be investigated under realistic conditions. We conducted a field experiment to delimit spatially the extent to which soil around natural Lumbricus terrestris burrows is influenced biochemically. We placed plant litter dual-labelled with ¹³C and ¹⁵N stable isotope tracers on L. terrestris burrow openings and we measured residue-derived ¹³C and ¹⁵N in thin concentric layers (0–2, 2–4, 4–8 mm) around burrows with or without a resident earthworm. After 45 days, earthworms were significantly enriched in ¹³C and ¹⁵N as a result of feeding on the plant litter. At 0–5 cm soil depth, soil ¹⁵N concentrations were significantly higher around occupied than unoccupied burrows, and they were significantly higher in all burrow layers (including 4–8 mm) than in bulk soil (50–75 mm from burrow). This suggests that biochemical drilosphere effects of anecic earthworms, at least in the uppermost portion of the burrow, extend farther than the 2 mm layer assumed traditionally.